Apple has updated the IOS for older iPhones which has resulted in a slower performance for those phones with degraded batteries. Apple has released a statement, " Our goal is to deliver the best experience for customers, which includes overall performance and prolonging the life of their devices." (CNN Tech Dec. 22, 2017). It would appear that older batteries are not up to the task up performing parallel tasks due to decreased capacity.
The only way to avoid this programming slowdown is to replace the degraded battery in your iPhone with a new, fresh one or just replace the entire phone. This intentional move by Apple was initially discovered by Reddit user TeckFire ran some CPU benchmarks (via Geekbench) on his iPhone 6 Plus before and after he replaced its battery. He found that CPU performance was significantly better after a battery replacement, which he attributed to Apple slowing down phones with low capacity batteries. - http://engt.co/2CZ8I9N